Group Title: Bulletin
Title: Uniformity of sprinkler and microirrigation systems for nurseries
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 Material Information
Title: Uniformity of sprinkler and microirrigation systems for nurseries
Series Title: Bulletin
Physical Description: 7 p. : ill. ; 28 cm.
Language: English
Creator: Haman, D. Z ( Dorota Z )
Smajstrla, A. G ( Allen George )
Pitts, Donald J ( Donald James )
Florida Cooperative Extension Service
Publisher: University of Florida, Cooperative Extension Service, Institute of Food and Agricultural Sciences
Place of Publication: Gainesville FL
Publication Date: 1997
Subject: Microirrigation -- Florida   ( lcsh )
Irrigation engineering -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical references (p. 7).
Statement of Responsibility: D.Z. Haman, A.G. Smajstrla and D.J. Pitts.
General Note: Caption title.
General Note: "June 1997."
 Record Information
Bibliographic ID: UF00008442
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6707
ltuf - ALP9637
oclc - 37865406
alephbibnum - 002296399

Full Text

Bulletin 321



Cooperative Extension Service
Institute of Food and Agricultural Sciences

Uniformity of Sprinkler and Microirrigation Systems for
D.Z. Haman, A.G. Smajstrla and D.J. Pitts2

Nursery production in Florida requires irrigation.
The majority of the systems found in container and
field nurseries are pressurized irrigation systems,
which are sprinklers or some type of microirrigation.
Most microirrigation systems are found in field
nurseries or in the larger, 3 gallon and more,
container production systems. The majority of
Florida nurseries, especially those that produce plants
in smaller containers, use overhead sprinkler systems.

Two terms which describe the performance of
irrigation systems are uniformity and irrigation system
application efficiency. A typical application efficiency
of a well-designed and managed sprinkler irrigation
system in a container nursery is only about 25% due
to necessary container spacing. This efficiency will be
further reduced by nonuniform water application.
For that reason, it is critical that the irrigation system
be designed for high uniformity and that this high
uniformity be maintained throughout the life of the


The coefficient of uniformity is an indicator of
how equal (or unequal) the application rates are
throughout the nursery. A low coefficient of unifor-
mity indicates that the application rates are very
different, while a high coefficient indicates that they

are very similar in value and the water is distributed
evenly to all plants.

Low coefficients of uniformity in sprinkler or
microirrigation systems can be due to numerous
factors, such as:

Inadequate selection of delivery pipe diameters
(submains, manifolds, and laterals).
Inadequate selection of sprinkler head and nozzle
in sprinkler irrigation or emitters in
Inadequate sprinkler overlap in sprinkler
irrigation, or too large spacings that are between
emitters in line source emitters in microirrigation.
Wind effects on sprinklers and microsprinklers.
Changes in system components with time, such as
changes in pump efficiency, pressure regulation,
or nozzle size.
Nozzle or emitter clogging.
Too high or too low operating pressure.


The performance of a sprinkler or microirrigation
systems can be evaluated by measuring operating
pressures, application rates, and the uniformity of
water application under nursery conditions.
Knowledge of these factors is important in order to

1. This document is Bulletin 321, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Publication date: June 1997.
2. D.Z. Haman, associate professor, A.G. Smajstrla, professor, Agricultural and Biological Engineering Dept., Gainesville, FL; DJ. Pitts, Assistant
professor, Southwest Florida Research and Education Center, Immokalee, FL., Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida, Gainesville FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research,
educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap,
or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / Christine Taylor Stephens, Dean

S'I ., r -" :-ler and Microirrigation Systems for Nurseries

determine the causes of poor uniformities. They also
F (~ -b allow for identification of changes in design,
installation, and maintenance which can improve
b- I water application uniformity. The above
measurements are also necessary for efficient and
effective irrigation system management. Field
SCIENCE evaluation of the system should be conducted at least
', annually to monitor changes in system performance
and to identify the needs for maintenance or repair.

Measuring Operating Pressures

It is important to monitor pressures at various
critical points in a irrigation system. A well-designed
and well-installed system will have permanent
pressure gauges at the critical points such as the
pump outlet, both sides of the filtration system, and
at the inlet to each irrigated zone. They allow the
system manager to monitor the performance of the
system and help to pinpoint any problem in the
system." For example, low pump discharge pressure
may be the result of pump wear, insufficient impeller
speed, excessive drawdown, or may be an indication
of problems downstream of the pump such as broken
pipe, too many zones running at the same time, or
excessive discharge from the nozzles. These gauges
should be checked periodically for proper functioning
and replaced as needed.

Sprinkler Irrigation Systems

A sprinkler is designed to operate efficiently over
a specific range of pressures, and its performance is
reduced at other pressures. Excessive pressures
produce very small droplets, resulting in fogging,
irregular rotation, and higher water application near
the sprinkler. Operating pressures that are too low
produce a doughnut-shaped spray pattern with very
little water near the sprinkler. The system uniformity
will definitely be affected if sprinklers aren't operated
within the range of pressures specified by the

Periodically, operating pressures within each zone
should be tested to evaluate system performance.
They can be measured at the sprinkler nozzles using
pitot tube attached to a pressure gauge. The pitot
tube should be placed about 1/8-inch from the nozzle
and adjusted until the highest constant pressure can
be read. This procedure is illustrated in Figure 1.
Pressures should be recorded at various points of the
irrigation zone with the close and distant sprinklers
included in the test.

Figure 1. Using a pitot tube to measure pressure at a
sprinkler nozzle.

Microirrigation Systems

Some microirrigation systems use pressure-
compensating emitters or have pressure or flow
regulation at the inlet to each lateral to minimize
pressure variation throughout the system. However,
most systems have pressure control only at the inlet
to the manifolds and use emitters with flow rate
dependent on the pressure within the line. In this
case, large variation in pressure can have a significant
impact on the uniformity of water distribution
throughout the system.

Pressures can be measured using a portable
pressure gauge equipped with a flexible tube and a
fitting which allows replacement of an emitter with a
gauge. The pressure distribution in the lateral line
with a large number of emitters (more than 10) will
not be significantly affected by blocking one emitter
while the others continue to flow. Some portable
pressure gauges are manufactured with a needle on a
flexible tube for direct insertion into the lateral line.
This method functions well for laterals constructed
out of the heavier materials with the walls at least
0.04 inches thick, which are typically used in nursery
systems. In these lines, the needle opening tends to
close when the needle is removed. In addition,
pressures should be measured at the inlet to the
laterals and at the end of laterals to determine
pressure drop along the lines.

Page 2

Uniformity of Sprinkler and Microirrigation Systems for Nurseries

Page 3

Measuring Application Rates

To schedule the irrigation events it is necessary to
know the sprinkler or emitter application rates. The
measurement of application rates under field
conditions will verify the design of the system.
Periodic measurements of the application rates will
also determine whether changes in application rates
occurred with time. This type of test should be per-
formed at least once each season.

Sprinkler Application Rates

The application rate for sprinkler irrigation
systems is expressed in inches/time. It refers to the
depth of water applied over an irrigated area during
an irrigation event. Basically, there are three
techniques to determine the water application rate of
a sprinkler system:

The water flow rate into the zone can be
measured and the application rate can be
calculated based on the area of this zone,
The application rate can be calculated from
measurement of the average flow rate and area
covered by each sprinkler, and
The application rate can be measured directly
with catch cans or rain gauges in the nursery.

It is recommended that each irrigation zone
includes a flow meter for monitoring the amount of
water applied during the irrigation event. This flow
meter can be used to determine the water flow rate
(gpm) to the zone. The flow rate per acre can be
calculated from the size of the zone (gpm/acre).
Based on the fact that each acre-inch is approximately
equal to 27,000 gal of water, the application rate
(inches/hr) can be determined (Example 1).

Example 1.

The flow rate to a 2-acre zone is 200 gpm. What
is the application rate in inches/hr to this zone? (1
acre-inch of water is approximately equal to
27,000 gal)

200 gal/min / 2 acres = 100 gpm/acre

100 gal/min-acre 60 m inches/hr /
27,000 gal/ac-in = 0.22 inches/hr

For regularly spaced sprinklers, the application
rate can be calculated from the average sprinkler

discharge and the spacing between the sprinklers
using Equation 1:

AR = 96.3 q / (S1 Sm)


AR application rate, expressed in inches/hr
q sprinkler discharge rate in gallons per
minute (gpm)
Sl sprinkler spacing along the lateral in ft
Sm Sprinkler spacing between laterals in ft

Example 2.

Using Equation 1, calculate the application rate
for sprinklers with a discharge of 3 gpm spaced on a
30 ft x 30 ft rectangular pattern.

AR = 96.3 3 / (30 x 30) = 0.32 inches/hr

Sprinkler discharge rates can be determined by
directly measuring the volume discharged per unit
time in the nursery. A flexible hose, which can be
slipped over the sprinkler can be used to redirect the
water into a graduated cylinder or other container of
known volume. Also, specification tables can be used
for predicting the sprinkler flow rates after measuring
the pressure at the nozzle with a pitot tube; however,
this applies only to new nozzles since the discharge
may change with nozzle wear. The nozzles should be
checked for wear or distortion with a drill bit having
the diameter specified for the nozzle.

The application rate can be also measured directly
with catch cans or rain gauges. The application rate
is the average depth measured per unit time of system
operation. It is recommended that 16 to 24 cans be
used for this measurement. A typical layout of the
cans is presented in Figure 2. These measurements
should be repeated in a few representative locations
of the nursery as demonstrated in Figure 3. Catch
cans should all be of the same size and shape, and
they should be located in a regular grid pattern and
clear from any vegetation. Preferably, they should be
placed close to the soil surface but if it is necessary,
they can be elevated above vegetation. A few drops
of light weight oil can be placed in the cans before
the test in order to reduce evaporation during the
tests. In Figure 2, the depth of measured water is
presented below each can. The numbers in
parentheses represent the absolute values of
deviation from the average measurement. These
values are used in the evaluation of system uniformity.

Uniformity of Sprinkler and Microirrigation Systems for Nurseries
~v__ ~\\



---------- --, ---

o O t o io
0.32' 0.34- 0.32- 0.34"
(.01) _J.03)_ _10 I03)

o 0 (D 10
0. 'h 0.28 0.25' 0.30"
(.o01) (.03) L (.06) (.01)
Catch Cans -1-. .---


k (.05)

Figure 2. Typical layout of catch cans for sprinkler
uniformity measurements.

Manifold Pipeline Sprinklers



Figure 3.

Example distribution of test locations in a large

Emitter Discharge Rates

Microirrigation systems apply water in a discrete
manner, without covering the entire area of the
nursery. It is much easier to discuss the discharge
rate for these system in terms of volume/time applied
by each emitter rather than an application rate in
inches/time as in overhead sprinkler systems. The
flow rates from emitters must be known for irrigation
scheduling and management. The tests should be

DU = (avg. low qtr depth
/ overall avg. depth) 100%

Example 3.

To demonstrate how Equation 2 is used, DU is
calculated for the measurements presented in Figure
2. The average of all measurements is 0.31 in.

DU = ((0.24 + 0.25 + 0.27 + 0.28)/4)
/ 0.31 100% = 83.9%

Another widely-used method is Christiansen's
Uniformity Coefficient which is expressed in Equa-
tion 3.

UC = (1-(avg. deviation from avg. depth
/ overall avg. depth)) 100%

Example 4.

Again, using measurements presented in Figure 2,
the average deviation can be calculated by averaging
the absolute values of the differences between each
measured depth and the average depth for 16 points
(0.46/16 = 0.029 in). This is divided by the overall
average depth (0.31 in). From Equation 3:


Page 4

performed to verify design and installation and
changes in application rates that may occur with time.
The techniques to perform these tests in the nursery
are similar to techniques for measurement of
discharge rates from the sprinkler nozzles. The
following measurements can be used to determine
emitter flow rates:

" Measuring water application into the zone and
calculating the rate of application from number of
emitters within the zone.
Collecting a known volume of water from
randomly selected emitters throughout the system
and calculating the flow rate by dividing the
volume by time in which it was collected.

Nursery Evaluation of Sprinkler Irrigation

Uniformity of water application with sprinkler
irrigation systems is usually reported as either the
Distribution Uniformity (DU) or Christiansen's
Uniformity Coefficient (UC). Distribution Uniformity
is based on the low quarter of irrigated area. This
implies that the lowest 1/4 of the measurements is
used for the calculations (Equation 2).

Uniformity of Sprinkler and Microirrigation Systems for Nurseries

Page 5

UC = (1- (0.029 / 0.31)) 100% = 92.7%

Normally, DU values are lower than the UC
values for the same set of data. For high value crop
and any system where chemicals are applied with
irrigation water, the uniformities should be high ( DU
greater than 80%, or UC greater than 87%). When
coefficients fall below the acceptable values for a
given system the repairs and adjustments should be
performed as soon as possible.

Field Evaluation of Uniformity of Nursery
Microirrigation System

Standards for the uniformity of water application
for microirrigation systems have been developed by
the American Society of Agricultural Engineers
(ASAE). Acceptable uniformity ranges for various
microirrigation systems are presented in Table 1.
These ranges represent the economically efficient
range of uniformities. Design for higher uniformities
than those shown will increase the initial system cost
without sufficient justification in plant performance
due to improved uniformity. However, designs for
lower uniformities will result in reduced production
and/or wasted water and fertilizes.

Table 1. Acceptable uniformities for microirrigation systems
designed to provide all plant's water requirements in times of
drought (ASAE).

Emitter Type Soil Slopes Uniformity Range
Point source on widely flat 90 95
spaced (greater than steep 85 90
13ft) plants
Point source on closely flat 85 90
spaced (less than 13 ft) steep 80 90
Line source on row flat 80 90
production steep 75 85

The uniformity must be higher for widely spaced
plants, where each individual emitter serves one plant
only. In production systems where each plant has
access to two or more emitters, the variation among
emitter discharges is less critical. Also, because
steeper slopes result in more costly designs, lower
uniformities are permitted to balance system initial
costs. The uniformity of water application can also be
improved by the use of more expensive, pressure
compensating emitters which are characterized by the
same flow rate under significant pressure variation.

Pressure compensating emitters are frequently used
where there is significant variation in topography of
the nursery or other conditions which make it difficult
to design the lateral lines within permissible flow

The method of uniformity evaluation for
microirrigation systems presented below provides a
quick nursery test which does not require any
specialized equipment. This test can be used by
irrigation system designers, installers, purchasers, or
managers. The method is based on statistical
evaluation. Uniformity of the irrigation system (U) is
defined in statistical terms (Equation 4).

U = 100% (1.0 V)

In this equation, V is the statistical coefficient of
variation which is a measure of the variability of the
individual emitter flow rates from the average emitter
flow rate. It is the standard deviation of the
individual flow rates divided by the average flow rate.
Since this equation expresses uniformity in relative
terms the method can be used regardless of the
magnitude of emitter flow rates.

The procedure requires a minimum of 18
measurements to evaluate an irrigation system or a
zone of an irrigation system. These 18 measurements
should include the extreme conditions encountered in
the system such as entrances to the laterals, distant
end, midpoints, etc. It is not necessary to measure
flow rates from the emitters tested. Rather, the time
to fill a specific container (a constant volume), such
as soft drink bottle, can be used. The time required
to fill the container can be accurately measured using
a watch with a second hand. Therefore, no special
equipment is necessary to perform this test of
uniformity at the nursery.

All 18 times required to fill the container should
be recorded. First, the largest 1/6 of the measure-
ments, T,, (in this case 3) should be added together.
Then, the smallest 1/6 of measurements, Tmin should
be added. These two numbers (Tma and Tin) are
used in Figure 4 to determine the uniformity of water

Example 5.

For the data listed in Table 2, we have 18
measurements of time. The three greatest times (1/6
of all measurements) recorded are 107 sec, 110 sec,
and 108 sec. The sum of these three times, Tm, is

Uniformity of Sprinkler and Microirrigation Systems for Nurseries

Table 2. Sample data set for Figure 1 example.

Data Point Pressure (psi) Measured Time (sec)
1 26 65
2 27 (high #2) 62 (low #1)
3 22 (low #3) 80
4 25 74
5 21 (low #1) 90 (high #1)
6 26 68
7 26 64 (low #2)
8 24 76
9 25 72
10 28 (high #1) 64 (low #3)
11 25 67
12 24 81
13 23 86 (high #3)
14 24 77
15 21 (low #2) 88 (high #2)
16 25 72
17 24 78
18 27 (high #3) 66

325 sec. The smallest times recorded were 89 sec, 91
sec, and 87 sec. The sum of these three
measurements, Tmin, is 267. From Figure 4, the
intersection of the vertical line drawn at Tmin and the
horizontal line drawn at Tma falls between 90% and
100% which can be interpreted as being an
"Excellent" uniformity.

The method presented above is a statistical
method. The degree of certainty that the values read
from Figure 4 are accurate changes depending on the
degree of variation among the measurements. If the
uniformity calculated from 18 samples is 90%, the
confidence limits for this uniformity are 3.5% (
Table 3). This means that we can be confident (with
95% certainty) that if we need 90% uniformity, the
actual field uniformity would be in the range of 86.5
to 93.5%. However, as indicated in Table 3, we are
less confident of the results when the uniformity is
low. If the first 18 measurements indicate low
uniformity, the number of measurements should
increase to 36 or 72 depending upon the calculated

0 50 100 150 200 250 300
Sum of the Lowest One-Sixth of Times, Tmin

Figure 4. Microirrigation field uniformity calculator based
on emitter flow rates.

Table 3. Confidence limits (95% level)
uniformity estimates.

on statistical

Uniformity Number of Samples Variability
US (%) 18 36 72 144 Vqs

90 3.5 2.4 1.7 1.2 0.1
80 7.3 5.0 3.4 2.4 0.2
70 11.2 7.8 5.4 3.8 0.3
60 16.2 10.9 7.6 5.4 0.4

uniformity. Note that Tm. and Tmin are then
calculated from 1/6 of the measurements which is 6
and 12, respectively.

Field procedure summary for evaluation of nursery
microirrigation system uniformity:

* Start the system and run it at its design operating
pressure long enough to purge air from the lines.

Measure the amount of time required for each of
18 (or more) emitters to fill a container. Be sure
that the emitters sampled represent all parts of
the irrigation system.

Compute Tma by adding 3 longest times (or 1/6
of the number of emitters measured) required to
fill the container.

Page 6

Uniformity of Sprinkler and Microirrigation Systems for Nurseries

* Compute Tmin by adding the 3 shortest times (or
1/6 of the number of emitters measured) required
to fill the container.

From Figure 4 determine the field uniformity at
the intersection of a vertical line drawn from Tmin
and horizontal line drawn from Tmx.

If the uniformity is too low or if the confidence
interval is too great, take more field data until the
desired confidence interval is obtained.


Smajstrla, A.G., B.J. Boman, G.A. Clark, D.Z.
Haman, D.J. Pitts and F.S. Zazueta. 1990. Field
Evaluation of Microirrigation Water Application
Uniformity. IFAS Ext. Bul. 265, University of
Florida, Gainesville, FL.

Smajstria, A.G., B.J. Boman, G.A. Clark, D.Z.
Haman, D.J. Pitts and F.S. Zazueta. 1990. Field
Evaluation of Irrigation Systems: Solid Set or
Portable Sprinkler Systems. IFAS Ext. Bul. 266,
University of Florida, Gainesville, FL.

ASAE. 1989a. Field evaluation of micro irrigation
systems. EP405.1. ASAE Standards. Amer. Soc.
Ag. Eng., St. Joseph, MI.

ASAE. 1989b. Design and installation of micro
irrigation systems. EP409. ASAE Standards.
Amer. Soc. Ag. Eng., St. Joseph, MI.

Smajstrla, A.G., D.S. Harrison and F.S. Zazueta.
1984. Field evaluation of trickier irrigation
systems: Uniformity of water application. IFAS
Ext. Bul. 195. University of Florida, Gainesville,

Merriam, J.L. and J. Keller. 1978. Farm Irrigation
System Evaluation: A Guide for Management.
Utah State University. Logan, UT. 271 pages.

Page 7

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